Normally the angel of death will come unannounced and separate the soul from the body... But in the case of the best of creation Prophet Muhammad (saw) the greatest man in history the Angel of Death approached the door of the Prophet (saw) asking for his permission to come to him. He has never asked for anyone's permission before, and so the Prophet (saw) said, let him come in... This is how loved he is by Allah (saw) and they dare to insult such a creation! Our Prophet! Our Honour!